The term "nitrogen control" describes the phenomenon whereby a limitation of the ammonia supply during growth of bacteria results in an increase in the synthesis of ammonia-assimilatory enzymes, some amino acid binding proteins, and some amino acid catabolic enzymes. Genetic studies with enterobacteria reveal that regulation via nitrogen availability is under the control of three regulatory loci: glnF, glnG, and glnL. Both positive and negative controls operate at the transcriptional level where the products of glnF and glnL appear to mediate the interconversion of the glnG product, NRl, from repressor to positive activator in response to nutritional conditions. However, neither the biochemistry of NRl activation nor the process by which the level of intracellular ammonia signals this interconversion is understood at present. For this reason, a study of the physiological parameters of the nitrogen control response was made in E. coli K12 using the level of glutamine synthetase (GS) as a measure of regulation. It was determined that some D-amino acids are capable of imposing a step-down physiology upon cells growing in medium containing excess ammonia. Furthermore, these studies show that, added in combination at 10 mM each where growth rate is not appreciably affected, D-gln, D-thr, D-lys, and gly generate an increase in the level of synthesis of GS at a rate equivalent to that imposed by ammonia-limited growth. The results of mutant studies indicate that adenyltransferase and PII play some role in the response to the D-amino acids whereas a positive response shows strict dependence upon the presence of the wild-type glnD allele. However, no correlation between a positive response and the state of adenylylation of GS can be made. Genetic experiments to assign map position to mutations affecting adenyltransferase and a unique constitutivity function have continued and the existence of genetic linkage between glnE and this gln "C" isolate has been excluded.